Hydroxyl functional binder and/or aspartate-based coating compositions are well known in the art. For example, EP 0 403 921 describes coating compositions with binders based on a polyisocyanate component and an isocyanate-reactive component containing specific secondary polyamines. These secondary polyamines are also called polyaspartic acid derivatives and are based on primary polyamines and diesters of maleic or fumaric acid. EP 0 470 461 also describes coating compositions for vehicle refinish applications containing a polyisocyanate component and an isocyanate-reactive secondary diamine prepared from 3,3′-dimethyl 4,4′-diamino dicyclohexylmethane and maleic diethylester.
A general disadvantage of the above aspartate-based coating compositions is that the component containing the curable aspartate remarkably discolours on storage. Clear coat components based on hydroxyl functional binders tend to discolour at storage, too. This yellowing and discoloring of the aspartate- and/or hydroxyl-binder-containing component is clearly not desired.
WO 2005/089085 discloses a hardener composition for use as a cross-linking component of a two-component polyurethane coating composition comprising a polyisocyanate, a catalyst for the curing reaction, a first antioxidant which is a sterically hindered phenol or a secondary aryl amine, and a second antioxidant which is an organophosphite. The combination of antioxidants stabilizes the color of a solution of a polyisocyanate in the presence of a catalyst.
U.S. Pat. No. 4,677,154 discloses a process for improving the color stability of polyurethanes, wherein the polyurethane reaction is conducted in the presence of an additive stabilizer comprising 2,6-di-(t-butyl)-p-cresol (BHT) and a compound selected from specific phosphites, diphosphonites, isocyanurates and other compounds such as di-stearyl thiodipropionate, di-stearyl pentaerythritol and 4,4′-methylene bis(2,6-di-t-butylphenol). The use of the additive package prevents discoloration of the thermoplastic polyurethane caused by discoloration of the polyisocyanate.
WO 2004/063242 discloses coating compositions containing aspartic compounds and polyisocyanate curing agents. The coating compositions contain a di-substituted phenol antioxidant or a hydroperoxide decomposer. Presence of the antioxidant or decomposer leads to coatings with good resistance to UV light.
However, it has not hitherto been possible to provide a satisfactory solution to avoid discoloration/yellowing of hydroxyl functional binder and/or aspartate-based clear coat components on storage.
Thus, there is accordingly still a need for hydroxyl functional binder and/or aspartate-based clear coat components which do not show discoloration and yellowing on storage at room temperature as well as at elevated temperatures, e.g., of up to 50° C. The clear coat components shall be color stable for a time period of at least 12 months, while at the same time good technological properties of applied films formed by a two-component coating composition system containing the curable clear coat component and a polyisocyanate cross-linking component shall be maintained. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.